Predictive Modeling for Glass-Side Laser Scribing of Thin Film Photovoltaic Cells

Abstract

Laser scribing of multilayer thin films is an important process for producing integrated serial interconnection of minimodules, used to reduce photocurrent and resistance losses in a large-area solar cell. Quality of such scribing contributes to the overall quality and efficiency of the solar cell and therefore predictive capabilities of the process are essential. Limited numerical work has been performed in predicting the thin film laser removal processes. In this study, a sequentially-coupled multilayer thermal and mechanical finite element model is developed to analyze the laser-induced spatio-temporal temperature and thermal stress responsible for SnO 2 :F film removal. A plasma expansion induced pressure model is also investigated to simulate the non-thermal film removal of CdTe due to the micro-explosion process. Corresponding experiments on SnO 2 :F films on glass substrates by 1064nm ns laser irradiation show a similar removal process to that predicted in the simulation. Differences between the model and experimental results are discussed and future model refinements are proposed. Both simulation and experimental results from glass-side laser scribing show clean film removal with minimum thermal effects indicating minimal changes to material electrical properties.

Removal Mechanism and Defect Characterization for Glass-Side Laser Scribing of CdTe/CdS Multilayer in Solar Cells

“…It can be seen that no other elements except Si and Sn in region B, which indicates that CdS and CdTe are completed removed in this region and no inter-diffusion of S into the TCO layer. The co-existing of Si and Sn is due to the larger electron penetration depths of SnO 2 :F compared to its thickness [13]. In region A (~10μm) -macro-crack broken area, elements of Cd and S are detected besides Sn and Si.…”

“…Therefore, understanding the film removal mechanisms and characterizing the potential defects during laser scribing processes are important for the cost reduction of thin film solar cells. In the previous work [13], P1 scribing of TCO layer has been studied. P2 and P3 are similar processes, since both are under micro-explosion mechanism with the plasma occurs at the interface between absorber and TCO layers.…”

“…To date, only rudimentary modeling efforts have been made, offering no predictive or optimization capabilities. Wang et al [13] developed two-dimensional thermal model to estimate the temperature distribution of CdTe-based thin film solar cells by ns laser pulse irradiated from the glass side, and simulate the film removal during micro-explosion process. However, the model does not consider the dynamic response of brittle materials during this high-velocity shock wave impact, and it also offers no experimental validations.…”

“…Although extensive work is carried out for optimizing the scribing qualities [4][5][6][7][8][9][10][11][12][13], the physical phenomena responsible for film removal during laser scribing and their effect on scribe quality are not well known. In addition, development of simulation capabilities will enable the fundamental understanding of the physical mechanisms and optimize the scribing processes instead of relying on trial-and-error experiments.…”

“…Micro-explosion processing is pronounced when the laser material penetration depth is much shallower than the film thickness, such as P2/P3 processes of CdTe and a-Si:H solar cells. Otherwise, films are removed through thermal ablation, such as P1 process for front contact films made by transparent conducting oxide (TCO) materials [13].…”